1. Field of the Invention
The present invention generally relates to semiconductor electronic switches, and more particularly to high power, high temperature gate-assisted turn-off thyristor devices, and methods of manufacturing the same.
2. Description of the Related Art
Thyristors are bistable power semiconductor devices that can be switched from an off-state to an on-state, or vice versa. Thyristors, along with other power semiconductor devices such as high-power bipolar junction transistors and power metal oxide semiconductor field effect transistors control or pass large amounts of current and block high voltages. Unlike conventional thyristors, a gate turn-off (GTO) thyristor is turned off by a reverse gate pulse. Generally, a trigger input implements conduction in GTO thyristors. Thereafter, the GTO thyristors behave as diodes.
A thyristor is a very rugged device in terms of transient currents, di/dt, and dv/dt capability. The forward voltage (Vf) drop in conventional silicon thyristors is approximately 1.5 V to 2 V, and for some higher power devices, it is approximately 3 V. Therefore, a thyristor can control or pass large amounts of current and effectively block high voltages (i.e., a voltage switch). Although Vf determines the on-state power loss of the device at any given current, the switching power loss becomes a dominating factor affecting the device function temperature at high operating frequencies. Because of this, the maximum switching frequencies attainable using conventional thyristors are limited, as compared with many other types of power devices.
Two important parameters for a thyristor are the built-in potential (which is a characteristic of any semiconductor material's bandgap) and the specific on-resistance (which is the resistance of the device in the linear region when the device is turned on). Preferably,, the specific on-resistance for a thyristor should be as small as possible so as to maximize the current per unit area for a given voltage applied to the thyristor. In particular, the lower the specific on-resistance, the lower the Vf drop is for a given current rating. Moreover, the minimum Vf for a given semiconductor material is its built-in potential (voltage).
Conventional thyristors may be made of silicon, for example, such as a silicon-controlled rectifier. However, thyristors made of silicon have certain performance limitations inherent in the silicon material itself, such as the thickness of the drift region. The largest contributory factor to specific on-resistance is the resistance of the thick low-doped drift region of the thyristor. Typically, as the rated voltage of a thyristor increases, the thickness of the drift region increases and the doping of the drift region decreases. Thus, the resistance of the drift region increases dramatically. Therefore, the thickness of the drift region should be minimized and the level of doping should be maximized, for any given rated voltage so as to minimize the specific on-resistance for the device.
Several thyristor structures have been developed in an attempt to solve the on-resistance problems described above. These conventional devices include variations of the silicon material to try to lower the on-resistance. However, the conventional devices are limited by the inherent characteristics of the silicon semiconductor material itself. The electrostatic breakdown field is lower in silicon than it is in silicon carbide. This, in turn, requires that these portions be physically thicker, which makes for a generally disadvantageous specific resistance.
Silicon carbide offers a number of advantageously unique physical and electronic properties, which makes it particularly useful for thyristors. This includes its high melting point, high thermal conductivity, radiation hardness (particularly to neutron radiation), wide bandgap, high breakdown electric field, and high saturated electron drift velocity. With the high breakdown field, thinner devices can be developed that block a given amount of voltage compared to silicon devices. This provides faster switching devices because a smaller volume of charge carrier must be removed during turn-off. Therefore, the power handling capability of silicon carbide GTO thyristors is much better than that of silicon GTO thyristors. Additionally, silicon carbide is physically rugged and chemically inert.
Because of the superior physical and chemical properties of silicon carbide, several applications for silicon carbide GTO thyristors exist including high voltage DC systems, traction circuits, motor control, power factor control, and other power conditioning circuits. Moreover, these systems may be found in electric or hybrid electric vehicles, including tanks and helicopters. However, one major limitations to the use of conventional silicon carbide GTO thyristors in these systems is the low turn-off gain. Turn-off gain is the ratio of the cathode current being switched off by the GTO thyristor divided by the maximum gate current required for the switching process. If the turn-off gain is low, then the switching losses will be high, thereby reducing the amount of power that a single silicon carbide GTO thyristor can handle. Also, the circuit providing the gate drive current to turn off the GTO thyristor requires extremely robust devices, which increases the cost and complexity of the overall system.
Conventional devices have achieved turn-off gains of 12.5 with a drift region thickness of 50 micrometers. However, as silicon carbide growth techniques improve, the thickened epilayers (epitaxially grown layers) will become less rough, and with this material quality improvement comes longer free carrier lifetimes and higher mobilities. This results in a disadvantageous lower turn-off gain.
Therefore, there remains a need for further development of silicon carbide GTO thyristors to overcome the deficiencies of the conventional devices. This includes increasing the turn-off gain and the maximum voltage blocked, and reducing the on-state resistance, among other needs.